The amount of torque an internal combustion engine must produce to compensate the rubbing friction of the powertrain is customarily referred to as the rubbing friction torque. In usual practice, the rubbing friction torque is measured for various operating conditions during engine calibration, and stored in a calibration table as a function of engine speed and powertrain temperature. During subsequent engine operation, the engine controller accesses the rubbing torque calibration table as part of its computation of engine output torque for various control algorithms such as electronic throttle control and electronic transmission control.
Measuring the rubbing friction torque is ordinarily accomplished by using an engine dynamometer to motor the engine under various combinations of speed and coolant temperature. Obviously, this can be a costly and time-consuming process, and can be performed for only a limited range of engine temperatures. Other methods are also possible, such as calculating the rubbing friction torque RFT during engine operation based on the relationship:       I    ⁢                  ⅆ        ω                    ⅆ        t              =            I      ⁢                           ⁢      T        -          P      ⁢                           ⁢      F      ⁢                           ⁢      T        -          R      ⁢                           ⁢      F      ⁢                           ⁢      T      where I is the engine moment of inertia, dω/dt is the engine acceleration, IT is the indicated torque of the engine, and PFT is the retarding torque due to pumping losses. But the moment of inertia I is difficult to estimate, and any estimation error is magnified by the engine acceleration which may be quite large. Although it is theoretically possible to obtain the test data at zero acceleration so that the term I(dω/dt) is zero, such test conditions are difficult to achieve as a practical matter. Accordingly, what is needed is a more comprehensive and cost-effective method for calibrating the rubbing friction torque of an engine.